1. Field of the Invention
The present invention relates to a piezoelectric actuator adapted for use in a camera and a shutter control device utilizing piezoelectric device.
2. Related Background Art
So-called piezoelectric actuator is a piezoelectric device used for converter an electrical signal into a mechanical displacement.
Such piezoelectric device consists of a stack of a plurality of thin ceramic plates having piezoelectric property, and, upon application of a voltage in the order of 100 V between two electrodes, shows a dimensional change by expansion or contraction depending on the polarity of said voltage.
Although the amount of displacement is limited to the order of tens of microns, such actuator is investigated as an electromechanical converter for small equipment such as camera, because it can generate a force as large as tens of kilograms despite of its limited volume.
The piezoelectric actuator with the above-mentioned features is utilized for driving a mechanical system, utilizing its expansion or contraction occurring between a state of no voltage application and a state of rated voltage application, but the available displacement and acceleration are limited and insufficient for a general driving source for mechanical systems.
Also such piezoelectric actuator is associated with a drawback of slow response, as it takes a certain time from the voltage application to the completion of displacement because of its mechanical inertia.
Besides, such piezoelectric actuator, constituting a capacitor in the equivalent circuit, absorbs a large current at the start of voltage application, and this current further increases and elevates the power consumption if the applied voltage is elevated in order to increase the amount of displacement. Thus the application of such piezoelectric actuator in a camera may reduce the service life of the battery therein, as the camera already includes large electric loads such as electronic flash unit and light source for illuminating liquid crystal display.
Also since the camera may be used in various postures, there may result a situation where the piezoelectric actuator employed in the camera cannot function properly even under the rated voltage application because of an excessive load, if the functioning direction of the actuator is opposite to the direction of gravity.
This drawback can be avoided by giving a sufficient margin to the maximum acceleration, achieved in the mechanical displacement available from the piezoelectric actuator, in consideration of the load. However, because the camera may be used in varied situations, ranging from underwater to space applications, it is not practical to design the piezoelectric actuator in anticipation of all these situations.
Furthermore, since certain fluctuations in performance are unavoidable in the piezoelectric actuator, in the mechanical loads and in the power supply circuit, the proper function may be hindered even under a proper voltage application, if these fluctuations appear simultaneously in the mutually aggravating manner.
Furthermore, eventual vibration or mechanical shock applied to the camera may also attenuate the force generated by the piezoelectric actuator, thus hindering the proper function thereof even under a proper voltage application.
Furthermore, the above-explained piezoelectric actuator has a drawback of being susceptible to humidity. The piezoelectric actuator is usually hermetically sealed with resin in order to exclude moisture, but the distance between the electrodes formed on both surfaces becomes small because of the multi-layered structure of thin ceramic plates and the insulation may be destructed by the voltage application in the order of 100 V under a high humidity condition. Such destruction of insulation may be temporary, but is in most cases permanent, whereby the piezoelectric actuator itself is destructed.
Since the camera is often used under high temperature and high humidity, the piezoelectric actuator employed therein is subjected to the same condition, and may cause destruction of insulation by humidity. This failure of the piezoelectric actuator is only noticed by the absence of start of an exposure operation when the user depresses the shutter button, so that he will lose the opportunity of phototaking.
Furthermore, such piezoelectric actuator is associated with various drawbacks which are related to a pyroelectric effect it usually exhibits. Said pyroelectric effect means a phenomenon of a voltage generation in the piezoelectric actuator by the infrared light or thermal radiation entering the actuator from the atmosphere.
When employed in a camera, the piezoelectric actuator generates a DC voltage between the electrodes thereof by said pyroelectric effect, by absorbing heat from the surrounding atmosphere in spontaneous manner, even while the voltage application by the depression of shutter button is not conducted. Although said voltage generation is gradual, it increases with time, thereby hindering proper function of the piezoelectric actuator at the voltage application thereto for a shutter releasing operation. If the voltage generated by the pyroelectric effect is same in polarity as the applied voltage, there will result an excessively large voltage, eventually leading to the destruction of piezoelectric actuator or undesirable effect on the associated electrical circuits. On the other hand, if the voltage resulting from the pyroelectric effect is opposite in polarity to the applied voltage, there will result an insufficient applied voltage.
Also the piezoelectric device in the actuator, having reversible characteristics, causes a mechanical displacement in response to a voltage application, and also generates a voltage when a mechanical pressure is applied from the outside. Because of this property, if the camera is given a mechanical shock or is dropped, a mechanical impact is given to the piezoelectric actuator to generate a large spike voltage, which flows to the associated circuits, thereby inducing destruction thereof or giving a noise to delicate control circuits.